Two-sweep signal transmission and reception in seismic exploration

ABSTRACT

This invention contemplates a method of seismic exploration using vibrators which are driven simultaneously by the combination of two distinct sweep signals. The combined signal is obtained, for example, by adding or subtracting the two distinct signals. The reflected seismic signals are detected in the usual fashion and recorded on a recording medium such as magnetic tape. If the number of seismic signals transmitted into the ground is an even number then the reflected seismic signals corresponding to each of said distinct signals can be obtained by summing, which includes adding and/or subtracting, the detected seismic signals.

United States Patent Holzman (4 1 June 20, 1972 [54] TWO-SWEEP SIGNALTRANSMISSION 3,416,632 12/1968 Bodine, Jr ..34o/|s.s TC AND RECEPTION INSEISNHC 3,332,512 7/1967 Undt ..340/ 15.5 EXPLORATION PrimaryExaminer-Benjamin A. Borchelt [72] inventor: Mark Holzman, Houston, Tex.Assistant Examiner-N. Moskowitz Attarne Michael P. Breston, Alan C.Rose, Walter R. Th 1 [73] Assignee: Western Geophysical Company ofAmerand Alfred Levine le ica, Houston, Tex. 22] Filed: May 4, 1970ABSTRACT This invention contem lates a method of seismic ex loration N 7p P {211 App] 0 using vibrators whichare driven simultaneously by thecombination of two distinct sweep signals. The combined signal is [52]US. Cl ..340/ 15.5 CP, 340/ 15.5 DP, 340/ 15.5 TC obtained, for example,by adding or subtracting the two [51] lnt.Cl. ..G0lv 1/22 distinctsignals. The reflected seismic signals are detected in 58 Field ofSearch ..340/155 PP, 15.5 TC the usual fashion and recorded on arecording medium such as magnetic tape. if the number of seismic signalstransmitted [56] References Cited into the ground is an even number thenthe reflected seismic signals corresponding to each of said distinctsignals can be UNITED STATES PATENTS obtained by summing, which includesadding and/or subtracting, the detected seismic signals. 3,332,5117/1967 Silverman ..34l/l5.5 TC 3,259,878 7/ 1966 Mifsud ..340/ 15.5 TC 4Claims, 1 Drawing Figure 22 I4 7 Z s 5 RECORDER COMPUTER 32 RECORDERCOMPUTER 4O E c o R 0 ER v a R ATO R l8 DETECTOR ARRAY ,x

TWO-SWEEP SIGNAL TRANSMISSION AND RECEPTION IN SEISMIC EXPLORATIONBACKGROUND OF THE INVENTION Vibrators are frequently used as energysources for land or marine seismic exploration. A vibrator transmitsenergy into the ground and is driven with an electric sweep signal whosefrequency is made to vary as a function of time over a predeterminedfrequency range. The reflected signal from each transmission is detectedby an array of detectors such as geophones or hydrophones. Thereafter,the vibrator is moved to a new transmission location, and thetransmission and reception steps are repeated. The number of signaltransmissions employed and the particular sweep frequency range utilizeddepend on the specific geologic structure being surveyed. To eliminateor minimize noise and to optimize the signal-tonoise ratio, it isconventional to sum the detected signals.

The frequency bandwidth of the sweep signal typically has a range fromsay 1050 Hz. Quite frequently, it may be desired or necessary to surveya particular geological area with two or more sweep signals. Forexample, it may be desired to survey the area with two sweep signalshaving respectively frequency ranges from 10-20 Hz and from 30-50 Hz. Ifboth sweep signals are used to consecutively energize the vibrator, thenan appreciable loss of valuable exploration time results. If one sweepsignal is used with a frequency range from 10-50 Hz, then the energy inthe 20-30 Hz band is wasted.

Additionally, since the energy of the transmitted signal is proportionalto the total sweep time T, then for a given constant sweep time T, theenergy in the 10-20 Hz band, when it is only a segment of the l-50 Hzband is much less than it would be if the 10-20 Hz sweep signal weretransmitted during the entire sweep time T.

The examples above given relate to two distinct bandwidths. In someinstances, it may be desired for the bandwidths to overlap: say, onesignal with a bandwidth between 10-40 Hz and the other between 30-50 Hz.

in practicing conventional methods, the required number of signaltransmissions is doubled when it is required to drive the vibrator withtwo distinct sweep signals. For example, the vibrator can be driven atone location with one sweep signal during a first time interval, andwith the other sweep signal during a second time interval. The vibratoris then moved to another transmission location to again becomeconsecutively energized by the two distinct sweep signals.

On the other hand, in practicing prior art methods, the vibrator couldbe driven by the first signal at each desired transmission location.Thereafter, the vibrator is driven with the other sweep signal at eachof the previous transmission locations. But, whether the vibrator isconsecutively driven by the two sweep signals at each transmissionlocation, or whether it is driven at each transmission location first byone sweep signal and later by the other sweep signal, there isengendered in each case a considerable loss of valuable time and/orenergy.

SUMMARY OF THE INVENTION In accordance with the present invention, thevibrator is energized at each transmission location by a combined sweepsignal which is either the sum or the difference of the individual sweepsignals. in order to recover the seismic reflections due to eachindividual sweep signal, the number of signal transmissions by thevibrator is limited to an even number. The detected seismic signals arethen suitably combined to obtain the reflections corresponding to theindividual sweep signals.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawing the sole FIGUREschematically illustrates the practice of the invention.

A mechanical vibrator 12, which is commercially available and welldescribed in US. patents, is driven by a combined sweep signal 14typically read from a magnetic tape in a time T, seismic energy whichbecomes reflected from subter-- ranean layers 19. The reflected energyis detected by an array of detectors 30, typically geophones orhydrophones, and converted by array 30 into an electric signal 32. Thedetected signal 32 is fed to a recorder 34 and thence to a computer 38.Digital computers are preferred because of their greater versatility andaccuracy. After the transmission and reception intervals, vibrator l2and array 30 are moved to their next respective positions and the abovesteps are repeated.

The practice of this invention will be better understood with referenceto specific examples. In these examples, it is assumed that it isdesired to operate the equipment at each of a number of vibrationlocations or points which will herein below be designated as VPs andthat the number ofsuch VPs in every instance will be even and thus equalto 2N where N is an integer.

For a first example, let the number of VPs be 2, and

S, Sweep signal 22 S Sweep signal 24 T, S, S Signal 14, for VP No. l,and

T s s,= Signal 14, for VP No.2 It thus follows that,

T, T, 28,, and

T2: 2S2 Hence the reflections from the individual signals can berecovered by suitably summing the received, detected signals 32.

Let R, be the received signal from VP No. l, and R be the receivedsignal from VP No. 2.

Then since the transmission of acoustic waves in the earth iseffectively linear, the summed signal (R, R will have the samecharacteristics as that which would have been produced from signal S,alone, and (R, R,) will be equated to the signal receivable from thetransmission of S alone. As a second example, let 2N= 6, and

T,=S,+S2 for VP No. l T =S,+S, for VP No. 2 'I" ,=S,+S2 for VP No. 3T4=S,S, for VP No. 4 T,=s,-s, for VP No. 5 T,,=S,S, for VP No. 6

If the result of transmitting S, is desired, then sum the correspondingdetected signals R,, R R

if the result of transmitting S is desired, then first subtract thedetected signals as follows:

a o a Q2 Then, sum the differences From the above examples, it can begeneralized that to sum 2N signals, vibrator 12 is driven by 2N/2 sumsweep signals and 2N/2 difference sweep signals. Then, to obtain theequivalent of the sum of the first sweep signals 8,, the detectedsignals are added; to obtain that of the sum of the second sweep signals8,, the detected signals are suitably combined, as illustrated in theexamples and the general formula. The various additions and subtractionsare accomplished by computer 38 and the results are recorded by recorder40 coupled to the output of computer 38.

it is to be understood that the proper crosscorrelation operator to beused in processing the signals obtained hereunder must be the signals Sand 8,. If signals S and S, are available, they may be used ascrosscorrelation operators without further refinement.

In normal recording practice it is, however, conventional to record theapplied sweep actually used in the field on a separate recording channelof the recording equipment 16. In such cases the individual sweepsignals S and S, are not readily available. They must, therefore, bereconstructed for use as crosscorrelation operators by applying to thesweeps the same equations which are hereinabove applied to the receivedsignals R. Thus in the general case the individual sweep signals may beobtained from the transmitted signals Zias recorded by the recordingequipment 16 in accordance with the following equations:

Accordingly from the foregoing it will be apparent that the presentinvention provides a novel method ofseismie vibratory prospectingwherein the seismic energy transmitted consists of the sum or thedifference of at least two sweep signals. An even number 2N of vibratorytransmissions is used to allow the separation and obtention of the sumof the two individual sweep signals. The method of the present inventioncan therefore be practiced in a minimum of time, with a minimum ofequipment and more economically than methods heretofore employed.

What I claim is:

l. A method of seismic exploration comprising the steps of:

selecting a first desired electric signal,

selecting a second desired electric signal,

forming an even number of composite electric signals by algebraicallysumming said first and second electric signals, converting eachcomposite electric signal to a composite acoustic signal,

transmitting each composite acoustic signal into the earth,

detecting each transmitted acoustic signal after it has travelledthrough a portion of the earth,

converting each detected signal into a corresponding detected compositeelectric signal, and

recording each detected composite electric signal.

2. The method of claim 1 and further including the step of:

algebraically summing the recorded detected composite electric signals.

3. The method of claim 2 wherein, the sum of additions and/orsubtractions of said recorded detected composite electric signalsprovides the sum of the detected electric signals resulting only fromsaid first electric signals.

4. The method of claim 2 wherein,

the sum of additions and/or subtractions of said recorded detectedcomposite electric signals provides the sum of the detected electricsignals resulting only from said second electric signal.

UNTTTD STATES PATENT @TTTQT v @ERTEIMQATE QT @REQTWN Patent No.3,6?1,932 Dated June 20, 1972 Inventor(s) Mark Zman It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 2, line 55, rewrite equation as follows:

T 5 K+l S2 Signed and sealed this 19th day of December 1972.,

(SEAL) Attest:

EDWARD MQFLETCI ERJRO 7 ROBERT GOTISCHALK Attesting Officer Commissionerof Patents USCOMM-DC 6O376-F'69 U S. GOVERNMENT PRINTING OFFICE: 19690-356-334.

FORM PO-105O (10-69) UNITED STATES PATENT OFFHIE v I (IERTIFIQATE OFORREUHN Patent No. 3,671,932 Dated June 20, 1972 Inventor(s) Mark zmanIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2, line 75, rewrite equation as follows:

1 s K+1 S2 Signed and sealed this 19th day of December 1972.

(SEAL) A tte's z y EDWARD M .FLETCHER, JR I ROBERT GCYITSCHALK AttestingOfficer Commissioner of Patents USCOMM-DC 50376-P69 1 U.S. GOVERNMENTPRlNTING OFFICE 1959 0-356-334,

FORM PO-105O (10-69)

1. A method of seismic exploration comprising the steps of: selecting afirst desired electric signal, selecting a second desired electricsignal, forming an even number of composite electric signals byalgebraically summing said first and second electric signals, convertingeach composite electric signal to a composite acoustic signal,transmitting each composite acoustic signal into the earth, detectingeach transmitted acoustic signal after it has travelled through aportion of the earth, converting each detected signal into acorresponding detected composite electric signal, and recording eachdetected composite electric signal.
 2. The method of claim 1 and furtherincluding the step of: algebraically summing the recorded detectedcomposite electric signals.
 3. The method of claim 2 wherein, the sum ofadditions and/or subtractions of said recorded detected compositeelectric signals provides the sum of the detected electric signalsresulting only from said first electric signals.
 4. The method of claim2 wherein, the sum of additions and/or subtractions of said recordeddetected composite electric signals provides the sum of the detectedelectric signals resulting only from said second electric signal.